A cyseteinyl-tRNA synthetase will be purified frm selenium-accumulator species of Astragalus and from non-accumulator species. The ability to use cysteine and selenocysteine as substrates will be compared. Similar studies with the methionyl-tRNA synthetase from these plants will also be undertaken. The ability of these enzymes from various sources to charge tRNA with the sulfur and selenium analogs will be investigated.